The present invention is generally directed to a method and apparatus for controlling ventilation rates and indoor air quality in a Heating, Ventilation and Air Conditioning (HVAC) system; more particularly, the present invention is directed to a method and apparatus for controlling outdoor air flow volume in an HVAC system using trace gas concentration sensing.
Indoor air quality is, without question, a topic of necessary importance. Not only are occupants of buildings, e.g., office spaces and the like, increasingly concerned about and aware of health issues, but technical associations have recently issued standards which specify minimum ventilation rates for acceptable indoor air quality. For example, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) has issued ASHRAE Standard 62-1989 entitled "Ventilation for Acceptable Indoor Air Quality." ASHRAE standards are generally established to assist industry and the public by suggesting safe practices, and conformance with them is completely voluntary. Nevertheless, such standards are developed under the auspices of ASHRAE and reflect a consensus reached by concerned interests with respect to the topics contained in the standards. Moreover, many ASHRAE standards, such as ASHRAE Standard 62-1989, are rapidly becoming incorporated into local building codes.
ASHRAE 62-1989 specifies alternative procedures for ensuring acceptable air quality indoors: a ventilation rate procedure and an indoor air quality procedure. Acceptable air quality is achieved in accordance with the ventilation rate procedure by providing ventilation air of a specified quality and quantity to a given space. Similarly, acceptable air quality is achieved within the space in accordance with the indoor air quality procedure by controlling known specifiable contaminants.
In greater detail, ASHRAE 62-1989 prescribes supply rates of acceptable outdoor air required for acceptable indoor air quality for residential, institutional and commercial facilities. As set forth in the Standard, the supply rates incorporate an adequate margin of safety and to account for health variations among people. These rates are a function of, inter alia, the type of environment (i.e., smoking lounge, office space) and the number of occupants. The ASHRAE outdoor air requirements for ventilation range from about 15 (for example, in office reception areas) to about 60 cubic feet per minute (CFM) per person (in, for example, smoking lounges) as being the minimum prescribed supply rate of acceptable outdoor air required.
Indoor air quality is also a function of many variables, including, inter alia, the quality of outdoor air, the configuration of enclosed spaces, the design and maintenance of the ventilation system, as well as the presence and strength of contaminant sources. A properly designed ventilation system must take all of these variables into account to maintain an acceptable level of indoor air quality.
There are many sources of space contaminants. Humans and their activities release a wide assortment of organic and inorganic chemicals. Personal care products, photocopy machines and other office equipment release chemicals. Growth of fungal material can release spores into the air. Standing water can allow harmful bacteria to multiply. Office furniture, partitions, paint, floor coverings and cleaning materials release chemicals into the air. Cracks in below-grade walls and floors can allow radon gas to enter the building.
The outdoor air itself can be a source of unwanted contaminants. This is particularly true for buildings located in large, congested metropolitan areas, or in cases where the outdoor air intake location is near loading docks, garages or near the building exhaust.
Most air source contaminants can be classified as one of the following: particulate matter, inorganic compounds, volatile organic compounds or microbes and their by-products. Particulate matter includes, inter alia, tobacco smoke, dust, and the like. Inorganic air contaminants include gases such as carbon dioxide, carbon monoxide, nitrogen dioxide, ozone or radon as well as particulates like asbestos or fiberglass. Volatile organic compounds include those generated by, for example, cleaning materials, personal care products, furniture, carpet, adhesives, paint and people.
As noted above, one acceptable mode of compliance with ventilation rate requirements is by measuring and controlling deleterious substances below safe levels (such as is provided with ASHRAE Indoor Air Quality Procedure). This strategy is, however, inherently very rigorous because it considers all contaminants and implementation is difficult because insufficient knowledge exists respecting safe concentration levels for the thousands of combinations of potential indoor contaminants. In addition, it is currently not cost effective to purchase sensors to monitor all of these contaminants.
Another acceptable mode of complying with ventilation requirements is to provide continuous measurement of the ventilation air flow rate so that it can be regulated to a specified rate. One known practice is to set a fixed minimum position for the outdoor intake air damper which is expected to ensure adequate flow regulation in all modes of operation. However, observed results are only marginally acceptable in some cases, such as in a constant volume fan system, and are unacceptable in most variable air volume systems.
Thus, to ensure that a specified amount of ventilation air is supplied to a space, measurement and closed loop control of the ventilation air flow rate is highly desirable. However, measurement of the ventilation air flow rate can be very difficult and expensive to implement accurately.
For example, the ventilation (i.e., outdoor) air flow rate can be measured through use of an air flow meter. The most common technique used to measure air flow is the pitot tube air flow station. Such stations generally incorporate a fixed array of pitot tubes. The pitot tubes in these stations sense the velocity pressure of the air as it passes around the tube; the corresponding air velocity is related to the velocity pressure in a known manner. For measuring outdoor air flow, these flow stations would typically be installed near the outdoor air intake where the maximum air velocity is generally less than 500 ft/min. Since the velocity pressure associated with 500 ft/min is only 0.016 in. w.c. (4 Pa), a very small error in the output of the differential pressure transmitter used to evaluate the pitot tube signal can cause a very large error in the calculated air velocity. For this reason, pitot tube stations are generally not viable for air velocities below 800 ft/min (4.1 m/s) unless exceptionally accurate differential pressure transmitters with auto-zeroing capabilities are installed. Unfortunately the high cost associated with this type of transmitter may be prohibitive in many installations. Thus, pitot tube air flow stations are unable to provide the required sensing accuracy when installed in a typical outdoor air intake configuration.
A less common type of air flow station utilizes an array of thermally sensitive resistors (heated thermistors) to measure air flow. If voltage is applied across a thermistor, the relationship between the air velocity, air temperature and the power dissipation can be determined. Such quantities are generally determined through the use of microprocessors. These stations are capable of accurately measuring air velocity below 500 ft/min. However, because of the high sensitivity of these devices at low air flow rates, low levels of turbulence can adversely affect the accuracy of the air flow measurement. Unfortunately, outdoor air intakes are typically very turbulent environments. Moreover, complicated field calibration procedures must be employed and it is often difficult to accurately compensate for the effect of changing air temperature. Thus, thermal air flow stations are not a very good solution to the problem of directly measuring the outdoor air flow rate accurately.
Alternatively, if the outdoor and return air streams are adiabatically mixed, conservation of mass and energy laws can be utilized to calculate ventilation air flow rate on the basis of measured temperatures of the outside air, the return air and the mixed air. For example, outdoor air flow rate (CFM.sub.oa) can be determined based on the temperature of the outdoor air (T.sub.oa), temperature of return air (T.sub.ra), temperature of mixed air (T.sub.ma), as well as the mixed air flow rate (CFM.sub.ma) in accordance with the following relationship: ##EQU1##
A typical fan system layout, as shown in FIG. 1, can be used to implement the foregoing equation. Transmitters or sensors for measuring T.sub.ra, T.sub.ma, T.sub.oa and CMF.sub.ma are advantageously positioned as shown in FIG. 1. However, when the difference between T.sub.ra and T.sub.oa becomes small, even small errors in the measurement of either T.sub.ra or T.sub.oa can cause very large errors in the calculated outdoor air flow rate. Inasmuch as temperature sensing errors of .+-. 1 degree Fahrenheit are the industry norm, unacceptable measurements result when the difference between the return and outdoor air temperatures is less than 10 degrees Fahrenheit. Most environments have significant time periods when the difference between the outdoor and return air temperatures is less than 10 degrees. Thus, a temperature based thermal energy balance is only a marginally acceptable strategy for calculating outdoor air flow rates.
Multi-point sampling probes are also known. For example, U.S. Pat. No. 3,369,405 issued Feb. 20, 1968 to Galegar discloses a sampling system for simultaneous sampling from multiple points. The samples are stored in separate containers, and then sequentially passed to the analyzer for analysis. Further, U.S. Pat. No. 4,090,392 issued May 23, 1978 to Smith et al. discloses an automatic gas analyzer system which provides for sequential analysis of a number of samples. A number of sample tubes and vent lines are utilized to ostensibly assure a fresh sample at a point near the sample analyzer in the form of an atomic absorption spectrophotometer. A multiplexer in the form of a three-way control valve is utilized to sequentially pass the samples to the analyzer. Also disclosed is the use of sample inputs for auto-zero and auto-span adjustment of the analyzer. An analogous system for gas sampling in large ducts and pipes is shown in U.S. Pat. No. 4,051,731 issued Oct. 4, 1977 to Bohl et al.
None of these systems, however, provide an accurate, indirect method and apparatus for measuring the flow rate of outdoor, ventilation air, such as is necessary, for example, to ensure compliance with indoor air quality standards. Thus, there exists a long-felt and heretofore unresolved need of providing an accurate and reliable method and apparatus for controlling ventilation rates and indoor air quality.